The present invention relates to a pneumatic tire, more particularly to a tread reinforcing structure being capable of reducing tire noise during running.
In order to improve high-speed durability of a pneumatic tire especially a radial tire for passenger cars, a single-layered full-width nylon band over a breaker is widely used. Here, a band means a cord layer whose cord angle is less than about 10 degrees usually less than 5 degrees with respect to the tire equator, and a breaker means a cord layer whose cord angle is more than 10 degrees usually more than 15 degrees.
Such a single-layered full-width band can reduce a road noise around a frequency of 250 Hz heard on the inside of a vehicle during running. By providing a single-layered full-width band made of a high tensile modulus aramid cord instead of the relatively low modulus nylon cord, such a road noise may be reduced more, but a pass-by noise heard on the outside of a vehicle during running tends to increase.
It is therefore, an object of the present invention to provide a pneumatic tire, in which the road noise and pass-by noise can be improved.
According to the present invention, a pneumatic tire comprises
a carcass extending between bead portions through a tread portion and sidewall portions,
a breaker disposed radially outside a crown portion of the carcass and made of cords laid at an angle of from 15 to 40 degrees with respect to the tire equator,
a band disposed radially outside the breaker and comprising a full-width band ply extending over the substantially overall width of the breaker and made of at least one cord forming a small angle with respect to the tire equator, wherein
a cord or cords of said full-width band ply existing in a tread crown region satisfy the following conditions Eq.1-Eq.5,
T greater than 0.8xe2x80x83xe2x80x83Eq.1 
0xe2x89xa6K+6.67Txe2x88x9221.7xe2x80x83xe2x80x83Eq.2 
0xe2x89xa645xe2x88x9210Txe2x88x92Kxe2x80x83xe2x80x83Eq.3 
0xe2x89xa6106.8xe2x88x9257.5Txe2x88x92Kxe2x80x83xe2x80x83Eq.4 
7xe2x89xa6Exe2x89xa613xe2x80x83xe2x80x83Eq.5 
wherein
the tread crown region is defined as a region centered on the tire equator and having an axial width of either 30 mm or 25% of the axial width of the breaker which is not less than the other,
the symbol xe2x80x9cKxe2x80x9d is an stretch-resistance index (N) of a cord which is defined as the product of the sectional area S in sq.mm and 2% modulus M in N/sq.mm of the cord divided by 100,
the symbol xe2x80x9cTxe2x80x9d is an stretch in % of the cord in the tire being in its natural state without being mounted on a wheel rim, and
the symbol xe2x80x9cExe2x80x9d is a cord count per 1 cm width of the full-width band ply.